Observation of high-frequency vibrational gap modes for KI:Cl- and KI:Br

Strong far-IR impurity-induced gap modes are observed just below the bottom of the optic phonon band in KI: one for $\rm Cl^-$ and the other for $\rm Br^-$. Both of these modes have been predicted using empirical potentials. The agreement between theory and experiment for the two $\rm Cl^-$ modes indicates that the impurity displacement is dominant for the higher-frequency one but of little importance for the lower-frequency pocket-like gap mode.